1. Field of the Invention
The present invention relates to a solid oxide fuel cell (SOFC), and particularly to a stack structure (a flat-plate stack structure) in which sheet bodies and support members for supporting the sheet bodies are stacked in alternating layers. The sheet body may also be referred to as the “single cell,” and the support member may also be referred to as the “interconnector.”
2. Description of the Related Art
Conventionally, a solid oxide fuel cell having the above-mentioned stack structure has been known (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 2004-342584). In this case, the sheet body can be a fired body in which a solid electrolyte layer formed from zirconia, a fuel electrode layer, and an air electrode layer are arranged in layers such that the fuel electrode layer is formed on the upper surface of the solid electrolyte layer and such that the air electrode layer is formed on the lower surface of the solid electrolyte layer. Hereinafter, the support member adjacent to the upper side of each of the sheet bodies may also be referred to as the “upper support member,” and the support member adjacent to the lower side of each of the sheet bodies may also be referred to as the “lower support member.”
The support member can be configured to have a plane portion and a frame portion, which is provided along the overall perimeter of the plane portion and whose thickness is greater than that of the plane portion. In this case, each of the sheet bodies is held between the upper and lower support members such that an overall perimetric portion of the sheet body is sandwiched between the frame portion of the upper support member and the frame portion of the lower support member. By employment of this configuration, the lower surface of the plane portion of the upper support member, the inner wall surface of the frame portion of the upper support member, and the upper surface of the fuel electrode layer of the sheet body can define a fuel flow channel to which a fuel gas is supplied. Similarly, the upper surface of the plane portion of the lower support member, the inner wall surface of the frame portion of the lower support member, and the lower surface of the air electrode layer of the sheet body can define an air flow channel to which a gas (air) that contains oxygen is supplied.
According to the above configuration, in a state in which the sheet bodies are heated to a working temperature of the solid oxide fuel cell (e.g., 800° C.; hereinafter, merely referred to as the “working temperature”), a fuel gas and air are supplied to the fuel flow channels and to the air flow channels, respectively, whereby the fuel gas and air come into contact with the upper surfaces and the lower surfaces, respectively, of the sheet bodies. As a result, electricity-generating reactions occur in the sheet bodies.
Meanwhile, when the fuel gas and air supplied to the fuel flow channel and the air flow channel, respectively, differ in pressure (hereinafter, referred to merely as having a “pressure difference”), the pressure difference acts on the sheet body as an external force oriented perpendicularly to a planar direction of the sheet body (hereinafter, may be referred to merely as the “planar direction”). As a result, the greater the pressure difference, the greater the extent to which the sheet body (particularly, a portion around the center (hereinafter referred to as a “near center portion”) of the sheet body) can be deformed.
In the above-mentioned stack structure, the pressure difference can vary among the sheet bodies due to, for example, the fact that the flow rate of gas supplied to the flow channel can vary among the sheet bodies. Accordingly, the quantity of deformation of the sheet body (particularly, a near center portion of the sheet body) can vary among the sheet bodies.
Additionally, in recent years, in order to, for example, reduce the size of SOFC and lower internal electrical resistance, an attempt to form very thin sheet bodies and support members (particularly, very thin sheet bodies) has been carried out. When a sheet body is formed very thin, a support portion of the sheet body (a layer for supporting the sheet body) also becomes very thin. Accordingly, for a given pressure difference, the quantity of deformation of the sheet body increases. That is, for a given variation in the pressure difference among the sheet bodies, the quantity of deformation of the sheet body varies to a greater extent among the sheet bodies.
When the sheet body (particularly, a near center portion of the sheet body) is deformed, pressure loss associated with flow of fluid through the flow channel varies, so that the gas flow rate varies. This can cause variation in electricity-generating characteristics of the sheet body. Therefore, in order for the SOFC as a whole to stably exhibit expected electricity-generating characteristics, it is preferred to restrain variation in the quantity of deformation of the sheet body among the sheet bodies caused by variation in the pressure difference among the sheet bodies. This is the reason for a demand for the sheet body to be unlikely to be deformed at a working temperature upon subjection to an external force induced by the pressure difference.